Enhanced Near Infrared Reflectance with Brilliant Yellow Hues in

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Enhanced NIR Reflectance with Brilliant Yellow Hues in Scheelite type Solid Solutions, (LiLaZn)1/3MoO4 - BiVO4 for Energy Saving Products Aju Thara T. R., P. Prabhakar Rao, Saraswathy Divya, Athira K.V Raj, and Sreena Thankom Sreedharan ACS Sustainable Chem. Eng., Just Accepted Manuscript • Publication Date (Web): 18 Apr 2017 Downloaded from http://pubs.acs.org on April 19, 2017

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ACS Sustainable Chemistry & Engineering

Enhanced NIR Reflectance with Brilliant Yellow Hues in Scheelite type Solid Solutions, (LiLaZn)1/3MoO4- BiVO4 for Energy Saving Products Aju Thara T. R., P. Prabhakar Rao*, Divya S., Athira K. V. Raj, Sreena T. S. Materials Science and Technology Division, CSIR-National Institute for Interdisciplinary Science and Technology (NIIST), Trivandrum – 695 019, India __________________________________________________________________ ABSTRACT: Enhanced NIR solar reflectance with interesting yellow hues in a new series of scheelite type solid solutions, [(LiLaZn)x/3Bi1-x][MoxV1-x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized via conventional solid state reaction (SSR) method and planetary ball milling assisted solid state reaction (PBM) method. The structural, morphology and reflectance (absorption) properties and coloring performance of the prepared compositions were analyzed by various advanced techniques. The solid solutions undergo a phase transformation from a monoclinic to a tetragonal phase. The compounds exhibit strong absorption in the UV and blue regions of the visible spectrum displaying high NIR reflecting intense yellow shades ranging from reddish to greenish. The yellow hue and NIR reflectance enhances by the morphological modifications through PBM method. Typically the pigment [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 displayed intense yellow colour (b* 86.63) with near infrared reflectance of 95% much better values than the commercial sicopal yellow. The applicability studies of these pigments on concrete cement block and metal sheet imparts good colouring performance with high NIR solar reflectance. Chemical and light resistance tests indicate their durability in the extreme weathering conditions. Thus the prepared compositions consisting of less toxic elements demonstrate sustainable use of the present pigments in exterior surface coating applications as energy saving products. Keywords: Yellow pigment; Morphology; NIR reflectance; Cool Colorants * Corresponding author. Tel.: + 91 471 2515311; Fax: + 91 471 2491712 Email ID: [email protected] (P. Prabhakar Rao)

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INTRODUCTION The sustainable development of human society will depend on how to solve the urgent resources and environmental issues. Environmental safety and energy crisis are two major globally facing problems.1As requirement increases in the world’s warmer regions, global energy utilization for air conditioning is continue to rise significantly and could have a major impact on climate change. Considering the performance of the buildings, lesser surface temperatures reduce the heat penetrating into the building and thereby decreasing the cooling loads while making more contented interior thermal environment. The roofing materials with higher solar reflectance (the ability to reflect sunlight) and higher thermal emittance (the ability to radiate heat) stay cool in the sun. Thus it reduces the cooling energy in air-conditioned buildings and increase the resident comforts in unconditioned buildings.2 Conventional NIR reflecting pigment coatings reduce the heat building up and minimize the use of cooling power systems in buildings, automobiles etc. which contributes energy savings, cost effectiveness and environmental security. The particle size of a pigment can influence their colours that favorably affect its reflectance properties.3Moreover; pigments with small particle size possess high surface areas making them useful for a variety of applications including coatings. Most of the yellow pigments, but not all, of the toxicity issues are associated with heavy or toxic metals such as cobalt, cadmium, chromium, lead, etc. Over thetime, these metals poison the body andmany of them known or suspected to be carcinogens. There are numerous literatures on new high NIR solar reflecting and environmentally friendly yellow pigments.4-9

Throughout history, various yellow ceramic pigments have been used: yellow of vanadium-zirconia, tinvanadium yellow, cadmium yellows, lead antimoniate (PbSbO3), etc.10 Molybdate based yellow pigments get wide attention today due to its non toxic behavior. The pigments like Y2Ce2-xMoxO7+δ, Y6-xSixMoO12+δ,Sm6-xZrxMoO12+δ, etc. are good eco-friendly


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alternatives to toxic yellow pigments having better colour strength compared to otherinorganic pigments based on rare earth molybdenum oxides reported earlier.11-14In comparison with the commercially existing colourants, the colour performance of these class pigments are poor. The yellow pigments with bismuth vanadate have grown considerably due to its rich yellow colour strength.15 These set of circumstances lead us to focus on bismuth vanadate (BiVO4). Synthetically prepared BiVO4 pigment has brilliant greenish yellow colour. Due to its nontoxic nature and photochromic property16-19 it is considered to be a promising alternative to toxic lead chromate and cadmium sulphide pigments in automobile and paint industry.The valence band (VB) of BiVO4composed primarily of O2p states,with Bi 6p states contributing to the bottom and V3d to the middle of the valance band.20 Molecular orbital theory suggest that the substitution of dopants into BiVO4 can either decrease or increase the band gap, depending on the preferred substitutional site and energetically favored crystal structure. BiVO4 has conventionally obtained by solid-state

reaction method that produces large irregular BiVO4 crystals due to its rapid crystal growth property. By controlling the morphologies of BiVO4 structures, such as sizes and shapes we can improve its properties.19 Based on these concepts earlier we have successfully synthesized brilliantyellow pigments Li0.10 La

0.10Bi0.8Mo0.2V0.8O4

for energy savingapplications7with NIR reflectance of

91% and b* 81.86. The synthetic methods play an important role in the chemical and physical properties of metal oxides. Numerous syntheses methods have gained attention for the preparation of fine pigment powders namely the sol gel, co-precipitation, hydrothermal, citrate gel,

evaporation

to

dryness

method

and

other

gel

combustion

methods.21-25So

forfurtherimprovement of the NIR reflectance and yellow hue, adopt some morphological modifications. To avoid the band gap altering due to particle size drop bring Zn to the lattice. In



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this work, BiVO4were synthesizedby the conventional solid-state reaction (SSR) method and planetary ball milling assisted solid-state reaction (PBM) method. Our work suggests that the pigmental as well as the reflective performance of (LiLaZn)1/3MoO4 doped BiVO4 is greatly dependent on the structure and the morphology.The new title pigmentsconsists of less toxic elements such as La,Zn, Mo, Bi, V26and their derived compositions are reported as ecofriendly pigments which make them sustainable use for these applications.15,25 Further, they perform interesting NIR solar reflectance as well as good yellow hues and so can be used in coating applications and makes valuable benefits. EXPERIMENTAL SECTION Materials and Methods. The pigments of the formula [(LiLaZn)x/3Bi1-x][MoxV1-x]O4 (x=0, 0.1, 0.2, 0.3, 0.4) were prepared by the conventional solid state reaction (SSR) route. Li2CO3 (99.998% purity, Sigma Aldrich), La2O3(99.9% purity, Acros Organics),ZnO(99.0% purity Sigma Aldrich) Bi2O3 (99.999% purity, SigmaAldrich), MoO3 (99.99% purity, Acros Organics) and V2O5 (99.9%purity, Acros Organics) were weighed in the required stoichiometric ratio and then were wet mixed in an agate mortar using acetone as the wetting reagent. Forcomparison, BiVO4 samples were also prepared. The mixed product was dried in an air oven at 100 °C for 1 h. This process of mixing and drying was repeats thrice to obtain a homogeneous product. The obtained mixture calcined at in a platinum crucible in an air atmosphere furnace. The furnace were programmed by increasing thetemperature initially at 10°C per minute up to the temperature (400−500 °C) and then, the heating rate was decreased to 5 °C perminute up to the desired temperature (600 °C). The samples weresoaked at 600 °C for 6 h. The calcined compounds were ground into fine powder for carrying further characterization.


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In Planetary Ball Milling assisted solid state reaction (PBM) method, stoichiometric mixture of Li2CO3 (99.998% purity, Sigma Aldrich), La2O3(99.9% purity, Acros Organics), ZnO (99.0% purity Sigma Aldrich), MoO3 (99.99% purity, Acros Organics), Bi2O3 (99.999% purity, Sigma Aldrich), V2O5 (99.9% purity, Acros Organics) were weighed in the required stoichiometric ratio. Then the mixture was placed in silica containers along with silica balls of 10 mm diameter as grinding media (balls to powder mass ratio = 10:1). Dry mechanical milling was carried out in a Fritsch Pulverisette - 6 planetary ball mill about 20 h by using a rotating disc speed of 250 rpm.Calcination conditions applied as that of the SSR route. The coating of the pigment over a concrete cement block and metal sheet have accomplished by a two-step process. Firstly, the concrete cement block and metal sheet surface is pre-coated with TiO2, a white pigment possessing high reflectance in both visible and NIR region. Next, the particular pigment is applied to the pre-coated substrate material. For this, the typical pigment has ultrasonicated for 10 min to ascertain the complete dispersion of the pigment particles in an acrylic–acralyn emulsion. The ratio of pigment to binder hastaken as 1:1 by weight. The obtained viscous solution has coated over the concrete cement block and metal sheet and then these have allowed drying in air. Characterizations: The crystalline structure of the calcined powders haveanalyzed by recording the X-ray powder diffraction pattern (XRD) using a PAN alytical X’pert Pro diffractometer having a Ni filtered Cu Kα radiation with an X-ray tube operating at 40 kV, 30 mA. Data were collected from 10 to 90° 2θ range with a step size of 0.016°. The morphology of the powder samples were performed by means of scanning electronmicroscopy (SEM) using a JEOL JSM-5600 LV SEM instrument operated at 15 kV. Energy dispersive X-ray spectroscopy (EDS) analysis and elemental mapping of the samples were analyzed using silicon drift detector



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X-MaxN attached with a Carl Zeiss EVO SEM. EDS elemental mapping was conducted using Aztec Energy EDS Microanalysis software. Particle size analysis of the powder samples are carried out by means of Beckman Coulter LS 13 320 Particle Size Analyzer.For this, thepigmentpowder is dispersed in the distilled water then sonicated at the speed of 20rpm. The absorbance and reflectance spectra of the samples werecarried out withUV−vis-NIR spectrophotometer (Shimadzu, UV-3600) using barium sulphate as a reference. Optical measurements were performed in the 220 to 2500 nm wavelength range with a step size of 2 nm. The measurement conditions were as follows: anilluminant D65, 10° complementary observer and measuring geometry d/8°. As a crystalline semiconductor, the optical absorption near the band edge follows the formula27 αhν = A(hν –Eg)n/2 ----------- (1) where α, ν, Eg and A are absorption coefficient, light frequency, band gap and a constant, respectively. n depends on the characteristics of the transition in a semiconductor, i.e. direct transition (n = 1) or indirect transition (n = 4). The energy gap of the samples werecalculated from the Tauc plots of the (αhν)2 versus photon energy (hν).The intercept of the tangent to the xaxis give a good approximation of the band gap energy.The color coordinates were determined by coupling analytical software (UVPC Color Analysis Personal Spectroscopy Software V3, Shimadzu) to the UV-3600 spectrophotometer. The color of the pigments was evaluated according to The Commission Internationale del’ Eclairage (CIE) through L*a*b* 1976 color scales (CIE-LAB 1976 color scales). In this system, L* is the lightness axis (L* is zero for black and 100 for white), a* is the green (−)/ red (+) axis, and b*is the blue (−)/yellow (+) axis. The parameter C* (chroma) represents saturation of the color andis defined as C* = ((a*)2 + (b*)2)1/2 and h° represents the hue angle. The hue angle, h° is expressed in degrees and ranges from 0° to


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360° and is calculated using the formula h° = tan−1(b*/a*). The infrared reflectance of the powdered pigment samples was measured with UV−vis-NIR spectrophotometer (Shimadzu, UV3600 with an integrating sphere attachment) using polytetrafluoroethylene (PTFE) as a reference in the 700 to 2500 nm wavelength range with a step size of 5 nm. The IR solar reflectance was calculated in accordance with the ASTM standard number G173-03.28 The IR solar reflectance is expressed as the integral of the percent reflectance times the solar irradiance divided by the integral of thesolar irradiance when integrated over the 700−2500 nm range as shown in the formula, మఱబబ

R=

‫׬‬ళబబ ௥(ఒ)௜(ఒ)ௗఒ మఱబబ

‫׬‬ళబబ ௜(ఒ)ௗఒ

----------- (2)

where r(λ) is the spectral reflectance obtained from the experiment and i(λ) is the standard solar spectrum (Wm−2nm−1) obtained from the standard. The NIR solar reflectance spectra were determined from ASTM Standard G173-03.28 Chemical resistance tests using acid/base solutions and water were performed. The typical pigment with best color was treated with 5% HCl, HNO3, NaOH or H2O and soaked for one hour with continuous stirring using a magnetic stirrer. The pigment powder was then filtered, washed with deionized water, dried and weighed. The light resistance test was carried out by exposing the pigment powder to the natural sun light. The difference in color of the pigment after exposure to sunlight for 70h was examined using the color parameters. The CIE color coordinates were measured and the color difference (∆E*ab) was calculated from the following equation. * ∆E ab = (∆L*) 2 + (∆a*) 2 + (∆b*) 2


- - - - - - - - - - - (3)


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RESULTS AND DISCUSSION X-ray Diffraction Analysis. The XRD patterns of [(LiLaZn)x/3Bi1-x][MoxV1-x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigment powders synthesized via conventional solid state reaction (SSR) and planetary ball milling assisted solid state reaction (PBM) methods are shown in Figure 1. The compositions x=0 and 0.1 crystallize with a pure and partial monoclinic phase with the space group I2/b and all the peaks are indexed according to the JCPDS database number 01-075-1866. Partial phase transformation occurred for x = 0.1 substitution from pure m-BiVO4 to a mixture of m-BiVO4 and t-BiVO4. This leads to an expansion in unit cell volume, increase in compressive lattice strain, conduction band edge uplift, and band gap widening.29The compositions with x = 0.2,0.3 and 0.4 form the tetragonal phase, which can be indexed as per the JCPDS card number 01-075-2481 with the space group I41/a. This shows progressive doping of Li+, La3+, Zn2+and Mo6+bringa structural transition from the monoclinic scheelite to the tetragonal scheelite type.3032

The splitting of the peaks at 18.5°, 35°, and 45° of 2θ is a characteristic of scheelite monoclinic

phase. The intense and sharp peaks in the diffraction patterns confirm the crystalline nature of the powders. This reveals that all the samples are homogeneous and the doping of Li+, La3+, Zn2+, and Mo6+ form solid solution in BiVO4. Minor peaks of V4O9 can be seen from x=0.2 onwards. In BiVO4, the V site bounded with four oxygen atoms forms a VO4 tetrahedron and Bi site bounded with eight oxygen atoms forms a BiO8 dodecahedron. The transition metal tetrahedral arrangements determine the structural difference of monoclinic and tetragonal scheelite forms. The interlinked transition metal tetrahedra were observed in tetragonal scheelite form, whereas they are isolated in the monoclinic scheelite form.33The ionic radii of Bi3+ is 0.117 nm (CN= 8) and V5+is 0.035 nm (CN = 4). The doping of Li+ (0.092 nm), La3+(0.116nm) and Zn2+(0.090 nm) into Bi3+ and Mo6+(0.041 nm)34into V5+ will results in small distortion in theVO4


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tetrahedron and BiO8 dodecahedron of the lattice due to the difference in ionic radius. The lattice volume increases with increasing dopant concentration fromx = 0 to 0.4 which signifies the formation of solid solution. The crystallite size calculated from the Debye–Scherrer formula, D = 0.9λ / β cos θ ----------- (4) where D is the crystallite size, λ is the wavelength of X-ray used, β and θ are the half width of the X-ray diffraction lines and half diffraction angle 2θ. The instrumental broadening was rectified using silicon as the external standard. The crystallitesize of BiVO4is found to be 63.84 nm, 36.5 nm in SSR and PBM routes respectively. In SSR route, the addition of dopants didn’taffect much change in the crystallite size.On doping with Li+, La3+, Zn2+, and Mo6+, an increasein crystallite size upto 69.81 for x = 0.3 and then decrease to 63.83 for x = 0.4 is observed in PBM route. The lattice parameters and crystallite size obtained at various dopant concentrations in both these methods are shown in Tables S1 & S2 in the supporting information.

Morphological Studies. The synthesis condition and treatment process affects the morphological feature of the material. Figure 2 shows the scanning electron micrographs of [(LiLaZn)x/3Bi1-x][MoxV1-x]O4 (x = 0, 0.1, 0.2, 0.3, 0.4) pigments synthesized using the SSR route. The microstructure explains the crystalline nature of the particles. Thus addition of (LiLaZn)1/3MoO4 causes reduction in the particle size of BiVO4. The particles are agglomerated and have sharp edges. There is a wide distribution of particle sizes of [(LiLaZn)x/3Bi1-x][MoxV1x]O4

(x = 0, 0.1, 0.2, 0.3, 0.4) solid solutionswith an average size of 1–8 µm prepared by SSR

method. The SEM micrographs obtained by PBM method are shown in Figure 3. Ball milling helps to maintain uniform particle size and shape. The particles are slightly agglomerated. SEM analysis shows that the morphology is almost spherical and average size of sample increases as concentration of BiVO4 increases. The particle sizes of all the compositions synthesized by PBM



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method vary from 0.7-1.5 µm.This variation in the morphology with respect to synthesis method has been found to improve the colour and reflective properties of the pigments which has been discussed in the later part of text. The EDS was used to further determinate the chemical compositionof the as-obtained pigments.

EDS

spectra

(Figure

S1

of

the

supporting

information)

of

atypical

[(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4 sample synthesized by two methods shows the presence of Bi,V, La, Zn, Mo and Oelements, with close approximation to the calculated value. The Li element is not detected due to going beyond the detection range of the instrument. X- ray mapping analysis of the classic [(LiLaZn)0.099Bi0.7][Mo0.3V0.7]O4pigment synthesized by PBM method shown in Figure 4also shows that the elements are uniformly distributed within the matrix. SEM EDS analysis confirms the close agreement between the stoichiometric and the actual composition. Thus, the structural and morphological analysis confirms that La3+, Zn2+ and Mo6+ has been effectively inserted into the BiVO4 lattice.

Particle Size Analysis. Mean particle diameter of all samples synthesized by both SSR and PBM routes and calcined at 600°C are shown in Table 1. The particle size agrees effectively with the results obtained from SEM. Compared to BiVO4 synthesized by SSR route, there is a noticeable difference in particle size and it discloses a mean diameter of 7.18µm (size of 90% particles

Enhanced Near Infrared Reflectance with Brilliant Yellow Hues in

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